The present invention relates to the field of telecommunication, and more specifically to a method for scheduling users in a mobile or wireless communication system using space division multiple access.
In a space division multiple access (SDMA) mobile or wireless radio communication system multiple users can be scheduled on the same resource when the users are sufficiently spatially separated. The spatial separation is used in order to direct the beams towards the different users so that the different beams are not interfering with each other. Such beams can be obtained when using a multiple antenna system at the transmitter. Different weights are applied to the different antenna elements enabling it to generate beams directed in the appropriate user direction.
Especially in Multiple Input Multiple Output (MIMO) systems at least two transmit antennas are used and separately encoded data signals can be transmitted per transmit antenna via two or more downlink beams of the downlink channel on the same resource.
For MIMO in LTE Release 8 (3GPP TS 36.213 V8.2.0) a codebook based precoding/beamforming will be used and single user MIMO feedback information comprises two parameters: CQI (CQI=channel quality indication) and PMI (PMI=precoding matrix indicator). CQI is a parameter indicating a channel quality of a downlink beam with a largest average received modulated carrier power. For LTE Release 8, the CQI parameter is an index for a transport format of the downlink channel. PMI is a parameter indicating a beam index, which corresponds to a precoding vector of the downlink beam for which the channel quality is reported. The beam index belongs to an entry of a codebook, which contains precoding vectors with all allowed Tx antenna weights combinations.
This situation is illustrated in FIG. 1 where the base station BS sends downlink reference signals (pilots) which are evaluated by the user terminal UE1. The user terminal UE1 determines then the best adapted precoding weights to be used based on a codebook shared between the base station and the user UE1 and determines the corresponding channel quality which would be experienced if the base station would send data using the selected precoding weights. Having done this evaluation, the user terminal sends on a feedback channel an indicator for the selected precoding vector (PMI) as well and an indicator for the corresponding channel quality (CQI). The PMI can be coded on a number of bits depending on the codebook size (a codebook comprising 16 precoding vectors would result on a 4 bit-PMI). Similarly a certain number of bits are used for transmitting the CQI.
The multi user MIMO technique uses multiple downlink beams (or precoding vectors), which are directed to multiple mobile terminals. It is beneficial for performance and overall data throughput in a radio cell or in a sector of the radio cell, if a downlink beam directed to a specific mobile terminal generates as few interference as possible to downlink beams directed to other mobile terminals. Therefore a base station may build a set of adequate mobile terminals which are using the same frequency/time resource. This means that the mobile terminals of such a set are scheduled at a same time with a same frequency but with spatial separation of downlink beams, so that the downlink beams generate fewest interference between each other.
A “best companion” approach aiming at associating 2 compatible precoding weights is illustrated in FIG. 2. In addition to the PMI CQI, user terminal UE1 further calculates the impact that a further transmission on the same resource would have on it in term of interference if a different precoding vector would be used during the transmission at the multiple antenna system. The user UE1 selects the precoding vector which would cause the least interference and further evaluates the level of interference that it would experience in case of a transmission by another User UEk using this best companion precoding vector. Both PCI and CQI are reported back on the feed back channel. The indicator of the best adapted precoding weights in case of transmission on the same resource (BCI) is reported together with the interference evaluation or more precisely the degradation on the channel quality (delta CQI) compared to the case where only one single transmission happens on the resource.
Having only the best companion reported is often not sufficient for an efficient scheduling. Especially, in case no user is located in the angular sector that would be covered by the beam corresponding to the BMI, this method is not sufficiently elaborated to provide good information on users which could be scheduled on the same resource. A solution could consist in reporting more than one best companion for example a list of possible companions with the corresponding channel quality degradation. This solution would however greatly increase the feedback amount which would be disadvantageous as this happens at the cost of the uplink data rate.
A particular object of the present invention is to provide for a solution to improve the relevant feedback information for being able to efficiently schedule users in SOMA network while keeping the amount of feedback at a reasonable level not to impact the uplink user data transmission.
Another object of the invention is to provide a base station and a mobile terminal adapted to support such mechanisms.